1. Field of the invention
This invention relates generally to laser detection and ranging ("LADAR") systems and, more particularly, to a method for increasing resolution of LADAR systems.
2. Description of Related Art
LADAR systems are finding many applications for locating and identifying objects including, in military environments, automatic target recognition ("ATR") systems. One such system is shown in FIG. 1. A laser signal 10 is transmitted by an optics package (not shown) on platform 12 to scan a geographical area called a scan pattern 14. Each scan pattern 14, which is sometimes referred to as a "footprint," is generated by scanning elevationally, or vertically, several times while scanning azimuthally, or horizontally, once. FIG. 1 illustrates a single elevational scan 15 during the azimuthal scan 18 for one of the footprints 14. Thus, each footprint 14 is defined by a plurality of elevational scans 16 such as the elevational scan 15 and the azimuthal scan 18. The principal difference between the successive footprints 14 is the location of the platform 12 at the start of the scanning process. An overlap 20 between the footprints 14 is determined by the velocity of the platform 12 in the direction of an arrow 21. The velocity, depression angle of the sensor with respect to the horizon, and total azimuth scan angle of the LADAR platform 12 determine the footprint 14 on the ground.
The laser signal 10 is typically a pulsed signal and may be either a single beam or a split beam. Because of many inherent performance advantages, split beam laser signals are typically employed by most LADAR systems. As illustrated in FIGS. 2A-2B, a single beam 22 may be split into several beamlets 24. The particular split-beam signal in FIGS. 2A-2B is formed by splitting a single beam 22 into a group 26 of seven beamlets 24, referred to collectively as a septet. The beamlets 24 are spaced apart from one another by an amount determined by the optics package (not shown) aboard the platform 12 transmitting the laser signal 10. This amount of separation is known as the "beam separation," and is referenced by numerals 25 and 27 in FIG. 2B. The azimuthal beam spacing 25 is the spacing between the individual beamlets 24 in the horizontal direction of arrow 33 in FIG. 2B. The azimuthal beam spacing 25 is determined by the beam splitter of the optics package (not shown). The elevational beam spacing 27 is the spacing between the groups 26 in the vertical direction of arrow 29 in FIG. 2B. The elevational beam spacing 27 between the groups 26 is determined by the rate of the elevational scan 16 (shown in FIG. 1) by the optics package (not shown) in the direction of arrow 29 in FIG. 2B.
Each pulse of the single beam 22 is split, and so the laser signal 10 transmitted during the elevational scan 16 in FIG. 1 is actually a series of grouped beamlets 24 like group 26. A single elevational scan of such a series of grouped pulses will be referred to, for present purposes, as a "nod." Nods are spaced apart from one another in a manner similar to groups 26. This separation shall be referred to as a nod spacing and is represented by the numeral 31 in FIG. 2B. Nod spacing 31 is determined by the rate of the azimuthal, or horizontal, scan 18 in the direction of arrow 33.
Returning to FIG. 1, the optics package aboard platform 12 transmits the groups 26 while scanning elevationally 16 and azimuthally 18. The scan pattern 14 therefore comprises a series of successive nods like elevational scan 15. Thus, the number of nods 15 in scan pattern 14 is determined by the rate of azimuth scan 18 rate and the number of beam groups 26 in any one nod 15 is determined by the rate of elevation scan 16. The elevation scan rate and the azimuth scan rate consequently influence the amount of information, or resolution, that can be obtained by a given optics package.
The laser signal 10 is continuously reflected back to the platform 12 which receives the reflected laser signal. The total return from each scan pattern 14 is known as a scan raster 14. Data is obtained from the received signal and processed. The type of processing will depend largely on the data obtained from the reflected laser signal 10 and the application to which the LADAR system is employed. For instance, the data may be processed to display an image or to provide image data for use in identifying an object detected within a scan pattern 14. The reflected signal is comprised of azimuthally spaced nods or groups of beamlets 24. The returned nods are combined to create a nod pattern comprised of pixels such as that illustrated in FIG. 3, each pixel corresponding to a single one of the reflected beamlets 24.
The resolution of data obtained from such a LADAR system results from several design trade-offs including how many pixels are needed on target to provide the automatic target recognition enough range information to autonomously identify targets. Other factors include the scan angles, the range, and the range accuracy of the system. The LADAR scan angle is determined by the velocity of the vehicle used to carry it. The faster the vehicle, the narrower the scan angle. For a given scan angle, the faster the platform 12 travels the greater the gap, or the smaller the overlap 20, on the ground between footprints 14. The scan angle is then set to provide a large ground coverage as well as sufficient overlap between scans to capture the desired target at the maximum vehicle speed. The LADAR range is influenced by the laser power, telescope collection aperture, detector response and system resolution. The range accuracy is influenced by the laser pulse widths and the pulse capture electronics.
A practical LADAR system design is based upon balancing several conflicting parameters. The ideal LADAR system would have high angular resolution, large scan angles, long range, a high range accuracy, and be very inexpensive. High angular resolution means that the angular spacing between pixels, i.e., reflected beamlets 24, is very small which results in many more pixels on the target of interest making it easier to "see." The larger the scan angles, the larger the area which can be searched for targets. The longer the range capability of the LADAR, the sooner the target can be found and the threat determined. Range accuracy is defined as how small of a range change can be resolved by the LADAR. The cost of the system is also frequently a major driver in the design. Each of these parameters are traded against the other to get the best system for the particular application.
However, it is desirable to still further increase the resolution of the data obtained from such a LADAR system. Further altering any one of the traditional design constraints will inherently alter the compromise achieved in any such design and so it is desirable to increase resolution without altering any of those factors. Still further, it is desirable to increase LADAR resolution without necessitating changes in the optics packages currently used in LADAR systems. Thus, there is a need for a new technique to improve LADAR resolution.